1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method using the image processing apparatus, in which a plurality of line sensors corresponding to different color components simultaneously scan different scanning positions that are arranged on an original document at intervals predetermined in a sub-scanning direction.
2. Description of the Related Art
An image processing apparatus, such as a scanner device and a copier, scans an original document printed on a sheet of paper, for example, in order to convert the document into digital data, store the image data, and output the stored image data. The image processing apparatus receives, by using line sensors, reflected light that has been irradiated onto the original document, and performs image processing on the generated scanned data based on an amount of the received light.
The image processing apparatus includes the line sensors arranged to separately scan a red component (R component), a green component (G component), and a blue component (B component). The image processing apparatus generates the image data based on a voltage value acquired from the line sensors.
Each line sensor of Red, Green, and Blue (RGB) is arranged at predetermined intervals in a sub-scanning direction to scan a different scanning position of the original document. Therefore, each line sensor simultaneously scans at different scanning positions on the original document. Normally, the scanned data acquired by each line sensor is sequentially accumulated in a scanned data memory. The scanned data acquired by each line sensor at the same scanning position is read out from the scanned data memory and combined to generate RGB format pixel data of each line. Furthermore, by combining the combined RGB format pixel data, the image data of the original document is generated.
When a sampling period of the line sensors is constant, a magnification at which the original document is scanned is determined in accordance with the relative scanning speed in the sub-scanning direction of the original document and each line sensor. For example, when enlarging the original document, the scanning speed is decreased, and when reducing the original document, the scanning speed is increased.
Thus, since the scanning speed is changed in accordance with the magnification at which the original document is scanned, depending on the scanning speed, each line sensor may not scan an image on the same line of the original document. Accordingly, when the scanning position of each line sensor is displaced, and the pixel data is generated by combining the scanned data acquired by each line sensor, a false color is generated at a boundary where the pixel data changes from black to white, thereby deteriorating the quality of the image. A technique for correcting the pixel data when displacement of the scanned line is generated due to a change of magnification (i.e., because the magnification is changed from 100 percent magnification) is well known.
As for a determination of an achromatic color and chromatic color, a first well-known method determines the achromatic color by acquiring a maximum value and a minimum value from a signal of each color on which analog/digital conversion has been performed, and by acquiring a threshold value based on two-dimensional distribution of the maximum value and the minimum value. When the maximum value equals the minimum value, the first method determines the achromatic color. Therefore, in the two-dimensional distribution of the maximum value and the minimum value, a range surrounded by an experimentally acquired threshold curve and a straight line of “maximum value=minimum value” is set to be in a range of achromatic color.
Based on scanned data scanned by a line sensor that scans a G component, a second well-known method performs linear interpolation processing on the scanned data of an R component and B component scanned by other line sensors. The linear interpolation processing is performed when the R component scanning line sensor and the B component scanning line sensor scan by using the number of offset lines that includes a decimal, in other words, when the intervals at which each line sensor scans are not equal to the intervals of an integer number of lines. More specifically, the linear interpolation processing is performed on the R component scanned data and the B component scanned data by using the two pieces of scanned data acquired by scanning by using the integer number of offset lines that is close to the number of offset lines including the number of decimals. Then, the R component scanned data and the B component scanned data on which the linear interpolation processing has been performed and the G component scanned data on which the linear interpolation processing has not been performed are combined.
At this time, since the linear interpolation processing is performed on the R component and the B component, but is not performed on the G component, a contrast difference of the R component and the B component with respect to the G component becomes large. Moreover, since the G component becomes magenta when converted into the three primary colors of cyan (C), magenta (M), and yellow (Y), the magenta of the G component having a large contrast becomes intense when the scanned data is combined.
Accordingly, when setting an area for determining the achromatic color in a saturation distribution diagram that is based on color difference data, the color is determined by enlarging the achromatic color determination area in a magenta direction.
However, in a case of an achromatic color determining method of either the first method or of the second method, the false color cannot be sufficiently restrained.
Moreover, in the achromatic color determination of the first method, since the threshold curve experimentally acquired in the two-dimensional distribution of maximum value and minimum value is changed in accordance with a change of magnification, various problems can occur. For example, in the case of an image processing apparatus that can change the magnification by one percent, the experimental number used to acquire the threshold curve increases and the type of the threshold curves also increases, thereby increasing a data amount of the threshold value stored in the image processing apparatus.
Additionally, the achromatic color determination of the second method requires a cumbersome process of performing the linear interpolation processing on the R component scanned data and the B component scanned data in accordance with the change of magnification based on the G component scanned data.